1. Field of the Invention
The present invention relates to a semiconductor memory device and, more particularly, to a technique of determining a timing at which a word line is selected, for example, when a level thereof is raised, using a row pre-decode signal.
2. Description of the Related Art
In a semiconductor memory device such as a dynamic random access memory (DRAM), a static random access memory (SRAM) or the like, a memory access operation is carried out as follows: first, a pre-decoding is carried out in advance by a row pre-decoder based on a combination of each logic level of a predetermined plurality of bits of an address signal; next, a row pre-decode signal is generated; next, a main decoding for selecting a word line is started by a row main decoder based on the row pre-decode signal and, at the same time, a word line driver is activated based on the row pre-decode signal; and thus, the main decoding is completed by the row main decoder, that is, a level of the word line to be selected is raised.
In this case, a predetermined wait time is needed from when the row pre-decode signal is output until the operation of the word line driver is started. This wait time corresponds to a time required until the row main decoder completes its main decoding in response to the row pre-decode signal. In a known conventional approach, the wait time is realized by a train of delay inverters. Concretely, the wait time is realized by suitably selecting the number of logic stages covering from the row pre-decode signal (wiring) to the word line driver on the chip.
According to this approach, however, it is difficult to always realize an optimum delay time due to nonuniformity of process, fluctuation of temperature, or the like. Therefore, where the wait time is too much shorter than a time originally needed, a problem occurs in that another word line different from a word line to be originally selected in a selection of a word line is erroneously selected. Conversely, where the wait time is too much longer than the time originally needed, another problem occurs in that the access time is unnecessarily prolonged.